Short-term changes of the feeding value of two ryegrass species as relatedto stage of maturity: Continuous digestion trials with sheep

Environmental influences can cause short-term changes in the feeding value of grasses during growth and maturation. Continuous digestion trials are ef fective in observing short-term effects, however, such trials are costly, t ime-consuming and labour-intensive. Therefore this study was conducted to e xamine the time interval that is necessary for a reliable assessment of sho rt-term changes of the organic matter (OM) digestibility of two ryegrass sp ecies, i.e., perennial ryegrass (Lolium perenne L., variety Gremie) and Ita lian ryegrass (Lolium multiflorum Lam., variety Lema). The two grasses were harvested daily between 23 April and 19 June during primary spring growth 1991 and each grass was offered to a group of five sheep for ad libitum int ake. The OM digestibility was calculated with the assumption that the feaca l output of day two represented the undigested nutrients of the grass eaten at day zero. The OM digestibility values were then calculated based on fou r different methods. Method 1 was based on daily values of OM digestibility . With method 2, rolling 3-day averages were calculated. Method 3 was based on the average OM digestibility values of successive 3-day intervals (1 - 3, 4 - 6,...). Method 4 was based on the averages of successive 7-day inter vals. An analysis of variance was conducted within each of the four methods using a two-factorial design (grass species and harvest day) including the interaction between the main effects. Grass species had no influence (P > 0.14) on OM digestibility in neither of the four methods of OM digestibility calculation. Although animals bad ad libitum access to feed, the OM digestibility values were on a high level. T he mean values over the whole experimental period were 80.6 and 79.4% for p erennial ryegrass and Italian ryegrass (method I), respectively. Irrespecti ve of method of calculation, harvest day affected (P = 0.0001) the OM diges tibility. For both grass species, the OM digestibilities declined with matu ration from 85% at the end of April to about 75% mid June. In general, nume rical differences between grass species were only small. Although OM digest ibility values were greater for perennial ryegrass than for Italian ryegras s, there were several occasions throughout the first growth when the OM of Italian ryegrass was more digestible than that of perennial ryegrass. These observations explain the interaction between grass species and harvest day that could be observed with methods 1, 2 and 3. No interaction effect (P = 0.4486) was observed with method 4. Based on the results of this trial, we conclude that the effect on the feeding value of ryegrass species of short -term changes in environmental conditions during primary spring growth can be determined reliably by continuous digestion trials based on 3-day interv al sampling schedules.